recipes: Drop remaining PR values from recipes

recipes: Drop remaining PR values from recipes

We've been removing PR values from recipes at upgrade time for a while. In general
anyone maintaining a binary distro would end up having to curate these themselves
so the values in OE-Core aren't really that useful anymore. In many ways it makes
sense to clear out the remaining ones (which are mostly for 'config' recipes that
are unlikely to increase in PV) and leave a clean slate for anyone implementing
a binary distro config.

References are left in meta-selftest since the tests there do involve them and
their removal upon upgrade.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>